This invention relates to high power static inverters that can be operated as a dc source having a controlled dc output voltage with an exemplary application to providing ice melting on electric power transmission lines and, when not so needed, providing reactive compensation to a transmission line.
Large voltage sourced inverters have been developed for use in the shunt compensation of electric power transmission systems (STATCOM)and other applications. Typically, these inverters operate with substantially constant dc terminal voltage and are connected to ac power systems through minimal tie reactance. The leakage reactance of the coupling transformer (typically 15%) may be the only tie reactance. If it is desired to connect a source or sink of real power to the dc terminals of such an inverter, then an additional dcxe2x80x94dc chopper is usually required to regulate the dc terminal voltage of the inverter down to the voltage level required by the dc load.
Recently, a unique application for high power dc power sources has emerged. In very cold conditions, ice buildup on ac transmission lines has caused serious problems. Under these conditions, it has been proposed to drive a large dc current through the (de-energized) transmission cables to cause enough heating to melt the ice. This application requires a very high power (hundreds of megawatts) dc supply with a dc voltage that can be regulated from zero to a specified maximum. A prior solution to this problem would be to use a line-commutated thyristor converter to deliver the controlled dc power to the load. Apart from other considerations, this solution is unattractive because the ice melting is rarely required and the thyristor converter would provide no useful function for the majority of the time.
There is a need therefore for an improved high powered dc source in which the dc voltage can be controlled from zero to a specified maximum
This need and others is satisfied by the invention in which a conventional STATCOM is modified for alternative use as a high-power voltage-regulated dc power source. This is an attractive solution for certain applications where the reactive power capability of a STATCOM is generally required, but the equipment would occasionally be required to operate instead as a large regulated dc power supply. It may also find use in other applications where a large regulated dc power supply is required with very high quality sinusoidal input current and very low ripple in the dc output voltage. The invention has particular relevance in providing the high dc power with a regulated dc voltage from zero to a specified maximum value required for melting ice on electric power transmission lines.
More particularly, the invention is directed to a dc power regulator for connecting an ac power supply to a dc load and controlling the voltage applied to the dc load. The regulator comprises an inverter. An input circuit including an inductance is connected between the ac terminals and the ac power supply. This inductance is scaled to maintain current drawn through the ac terminals below a rated maximum level for the inverter when the voltage at the ac terminals is zero. An output circuit connects the dc terminals of the inverter to the load and a controller controls the inverter to generate a selected dc voltage at the dc terminals of the inverter.
The regulator can comprise a first switch device shunting the inductance when closed, and the output circuit includes a second switch device disconnecting the dc load from the dc terminals of the inverter when open. The controller can operate the inverter to provide reactive compensation to the ac power supply with the first switch device closed and the second switch device open.
Another aspect of the invention is directed to apparatus for melting ice on an electric power transmission line using power from an ac transmission system. An input circuit comprises a coupling transformer connected in shunt with the ac transmission system, and an inductance connected in series with the ac terminals of an inverter and the coupling transformer. Again, the inductance is scaled to maintain current drawn through the ac terminals of the inverter below a rated maximum level for the inverter when voltage at the ac terminals is zero. An output circuit connects the dc terminals of the inverter to the electric power transmission line and a controller controls the inverter to produce a dc voltage at the dc terminals selectively variable between about zero and a maximum dc value.
The invention further embraces a method of providing dc power to a load from an ac transmission system by connecting the load to the dc terminals of an inverter, connecting the ac terminals of the inverter to the ac transmission system through an inductance having a value sufficient to maintain the current drawn through the ac terminals below a rated maximum level for the inverter when the voltage at the ac terminals is zero, and operating the inverter to produce a dc voltage at the dc terminal which varies from about zero up to a maximum dc value. Operating the inverter can comprise phase adjustment of the ac voltage at the ac terminals of the inverter relative to voltage on the ac transmission line or generating dc voltage at the dc terminals as a function of the magnitude of the ac voltage and adjusting the function to adjust the magnitude of the dc voltage. Alternatively, the dc load can be disconnected from the dc terminals, the inductance can be shunted, and the inverter operated to provide reactive compensation to the ac transmission system.
The invention further embraces a method of melting ice on an electric power transmission line using power from an ac transmission system by: connecting ac terminals of the inverter to the ac transmission system, connecting dc terminals of the inverter to the electric power transmission line, operating the inverter to generate a controlled dc voltage at the dc terminals that increases to a selected maximum value and then decreases towards zero, and limiting current provided to the ac terminals of the inverter from the ac transmission system to a rated maximum level. When the inverter is not needed to melt ice, the dc terminals can be disconnected from the electric power transmission line, the inductance can be shunted and the inverter can be operated to provide reactive compensation to the ac transmission system.